class Watcher{
    constructor(vm,key,cb){
        this.vm = vm
        this.key = key
        this.cb = cb

        Dep.target = this
        this.oldValue = vm[key]
        Dep.target =null
    }
    update (){
        let newValue = this.vm[this.key]
        if(this.oldValue === newValue){
            return
        }
        this.cb(newValue)
    }
    
}